Hydraulic drive circuits for use in hydraulic machines such as hydraulic excavators and cranes each include at least one hydraulic pump, at least one hydraulic actuator driven by a hydraulic fluid delivered from the hydraulic pump, and a flow control valve connected between the hydraulic pump and the actuator for controlling a flow rate of the hydraulic fluid supplied to the actuator. It is known that some of those hydraulic drive circuits employs a technique called load sensing control (LS control) for controlling the delivery rate of the hydraulic pump. The load sensing control is to control the delivery rate of the hydraulic pump such that a delivery pressure of the hydraulic pump is held at a fixed value higher than a load pressure of the hydraulic actuator. This causes the delivery rate of the hydraulic pump to be controlled dependent on the load pressure of the hydraulic actuator, and hence permits economic operation.
Meanwhile, the load sensing control is carried out by detecting a differential pressure (LS pressure) between the delivery pressure and the load pressure, and controlling the displacement volume of the hydraulic pump, or the position (tilting amount) of a swash plate in the case of a swash plate pump, in response to a deviation between the LS differential pressure and a differential pressure target value. Conventionally, the detection of the differential pressure and the control of tilting amount of the swash plate have usually been carried out in a hydraulic manner as disclosed in JP, A, 60-11706, for example. This conventional arrangement will briefly be described below.
A pump control system disclosed in JP, A, 60-11706 comprises a control valve having one end subjected to the delivery pressure of a hydraulic pump and the other end subjected to both the maximum load pressure among a plurality of actuators and the urging force of a spring, and a cylinder unit operation of which is controlled by a hydraulic fluid passing through the control valve for regulating the swash plate position of the hydraulic pump. The spring at one end of the control valve is to set a target value of the LS differential pressure. Depending on the deviation occurred between the LS differential pressure and the target value, the control valve is driven and the cylinder unit is operated to regulate the swash plate position, whereby the pump delivery rate is controlled so that the LS differential pressure is held at the target value. The cylinder unit has a spring built therein to apply an urging force in opposite relation to the direction in which the cylinder unit is driven upon inflow of the hydraulic fluid.
However, the above conventional control system for the hydraulic pump has had problems below.
In the conventional pump control system, the tilting speed of a swash plate of the hydraulic pump is determined dependent on the flow rate of the hydraulic fluid flowing into the cylinder unit, while the flow rate of the hydraulic fluid is determined dependent on both an opening, i.e., a position, of the control valve and setting of the spring in the cylinder unit and, in turn, the position of the control valve is determined by the relationship between the urging force of the LS differential pressure and the spring force for setting the target value. Here, the spring of the control valve and the spring of the cylinder unit each have a fixed spring constant. Accordingly, a control gain for the tilting speed of the swash plate dependent on the deviation between the LS differential pressure and the target value thereof is always constant. The control gain, i.e., the spring constants of the two springs, are set in such a range that change in the pump delivery pressure will not cause hunting and the pump is kept from coming into disablement of control on account of change in the delivery rate upon change in the swash plate position.
In the LS control, the delivery pressure of the hydraulic pump is determined dependent on a difference between the flow rate of the hydraulic fluid flowing into a line, extending from the hydraulic pump to the flow control valve, and the flow rate of the hydraulic fluid flowing out of the line, as well as a volume into which the delivered hydraulic fluid is allowed to flow. Therefore, when the operation (input) amount of the flow control valve (i.e., the demanded flow rate) is small, the opening of the flow control valve is so reduced that the small line volume between the hydraulic pump and the flow control valve plays a predominant factor. As a result, the delivery pressure is largely varied even with slight change in the flow rate upon change in the swash plate position. On the other hand, when the operation amount of the flow control valve is increased to enlarge the opening thereof, the large line volume between the pump and an actuator now takes part in pressure change, whereby change in the delivery pressure upon change in the delivery rate is reduced.
Accordingly, in order to prevent the occurrence of hunting over a range of the entire operation amount (opening) of the flow control valve, the above-mentioned control gain, i.e., the spring constants of the two springs, are set to provide such a tilting speed of the swash plate as to prevent the pressure change from hunting at the small opening of the flow control valve for the positive LS control.
With the control gain set as explained above, under a condition that the operation amount of the flow control valve is small and hence its opening is small, i.e., when the hydraulic pump is at the low delivery rate, change in the delivery rate produce proper change in pressure and will not cause hunting. But under a condition that the operation amount of the flow control valve is large and hence its opening is large, i.e., when the hydraulic pump is at the high delivery rate, the tilting speed of the swash plate dependent on change in the delivery rate is restricted by the above-mentioned control gain, and too small pressure change makes it difficult to control the delivery pressure with a good response. For instance, therefore, when an operating lever of the flow control valve is operated in a large stroke to increase the opening of the flow control valve, an operator is forced to feel that the actuator is too slow in action.
Further, when the operating lever is operated at small speeds and hence the deviation between the demanded flow rate of the flow control valve and the delivery rate of the hydraulic pump is small, the deviation between the LS differential pressure and the differential pressure target value is also small, and thus the change in pressure upon change in the tilting speed of the swash plate, i.e., the change in the delivery rate is sufficient to realize demanded speed change of the actuator. On the contrary, when the operating lever of the flow control valve is operated at large speeds to abruptly increase the opening of the flow control valve, there occurs a large difference between the demanded flow rate of the flow control valve and the delivery rate of the hydraulic pump, which also increases the deviation between the LS differential pressure and the differential pressure target value. Under this condition, the tilting speed of the swash plate is restricted by the above-mentioned control gain, and hence it takes a time for the once reduced differential pressure to return to its target value. As a consequence, the demanded speed change of the actuator cannot be realized, causing the operator to feel that the actuator is too slow in action.
The above description has been made without taking into account a revolution speed of the hydraulic pump. The delivery rate of the hydraulic pump is also influenced by the pump revolution speed such that when the pump revolution speed is high, even slight change in the swash plate position produce large flow rate change and hence large pressure change. In construction machines such as hydraulic excavators, a hydraulic pump is driven by a prime mover via a speed reducer and, as a revolution speed of the prime mover changes, a pump revolution speed is also changed. It is hence required that change in the flow rate dependent on change in the swash plate position be kept within a proper range even at the maximum pump revolution speed, in order to prevent the occurrence of hunting over an entire range of the pump revolution speed, i.e., the revolution speed of the prime mover, and to ensure the positive LS control. For this purpose, the above-mentioned control gain, i.e., the spring constants of the two springs, are also so set as to prevent the pressure change from hunting at the maximum pump revolution speed (or the revolution speed of the prime mover).
With the control gain thus set, when the revolution speed of the hydraulic pump is at maximum, change in the swash plate position produces satisfactory change in the delivery rate to realize the demanded speed change of the actuator. However, when the pump revolution speed is low, the tilting speed of the swash plate is restricted by the above-mentioned control gain, and change in the swash plate position produces small change in the delivery rate. Consequently, the demanded speed change of the actuator cannot be realized and the operator is forced to feel that the actuator is too slow in action.
An object of the present invention is to provide a control system for a hydraulic pump which permits, in a hydraulic drive circuit of load sensing control type, to properly control a change rate of the delivery rate with respect to change in the displacement volume of the hydraulic pump to prevent the occurrence of hunting due to an abrupt change of the pump delivery pressure and achieve a prompt response.